The terahertz wave is generally an electromagnetic wave of a frequency from 0.1 THz to 30 THz. The terahertz wave is expected to be developed from the basic science field such as physical property, electron spectroscopy, life science, chemistry, pharmaceutical science, and the like to the application field such as atmospheric environmental measurement, security, material inspection, food inspection, communication, and the like.
For example, there have been expected applications of the terahertz wave to an image diagnosis apparatus which non-destructively diagnoses (inspects) an object in order to utilize characteristics that photon energy is small and the frequency is higher than that of a microwave and a millimeter wave. Particularly, because its wavelength range includes an absorption wavelength peculiar to a constitutive substance of a biological cell, there have been expected applications of the terahertz wave to an apparatus that can inspect and observe the biological cell in real time. Conventionally, inspection and observation of the biological cell cannot be performed without dyeing because of using a pigment. Therefore, it has taken time and labor for the inspection and the observation. For example, there is already publicly known an apparatus that can observe by utilizing the terahertz wave, a cell sample which it is difficult to observe by visible light (refer to Patent Document 1, for example).
In the apparatus disclosed in Patent Document 1, an electro-optic single crystal is used as a detection element of the terahertz wave. Specifically, there is used a characteristic of an electro-optic single crystal that a refractive index changes in accordance with the intensity of an incident terahertz wave. The change in the refractive index can be detected as a change in a phase, polarization, and intensity (a light quantity) of light, when the light such as infrared light (referred to as detection light, probe light, and the like) is irradiated in superposition to an electro-optic single crystal to which the terahertz wave is being irradiated. In the apparatus disclosed in Patent Document 1, a terahertz wave having a spatial distribution generated in the intensity (spatially modulated intensity) due to transmission through a specimen is incident to the electro-optic crystal. A spatial distribution of a refractive index change generated in the electro-optic single crystal in accordance with the intensity distribution is read as a light quantity distribution of near-infrared light. By this arrangement, the specimen can be observed.
In an observation apparatus that performs observation based on this principle, in order to obtain high spatial resolution, it is required to thin the electro-optic crystal as much as possible such that the terahertz wave transmitted through the specimen does not spread due to the influence of diffraction. In Patent Document 1, there is also disclosed a terahertz-wave detection element in which the electro-optic crystal is supported by a reinforcing member, by having the electro-optic crystal itself formed extremely thin.
On the other hand, there is also already publicly known a terahertz electromagnetic wave detector that uses a ZnTe crystal of a thickness equal to or larger than 5 μm and equal to or smaller than 100 μm as the electro-optic crystal, in order to reduce the influence of a multiple reflection and expand a measurable terahertz band (refer to Patent Document 2, for example). According to a technique disclosed in Patent Document 2, a ZnTe crystal is also used in a supporting substrate that supports the electro-optic crystal, and both crystals are joined together by thermocompression.
Further, there is also already publicly known an adhered body having a lithium niobate single crystal or a lithium tantalate single crystal as the electro-optic crystal equal to or larger than 0.1 μm and equal to or smaller than 10 μm and having a supporting substrate adhered thereto by a resin having a fluorene skeleton (refer to Patent Document 3, for example).
As described above, in order to obtain high spatial resolution in the observation apparatus using the terahertz wave, it is required to thin the electro-optic crystal used for detection. In order to realize this, a terahertz-wave detection element is usually manufactured by thinning an electro-optic crystal after the electro-optic crystal and a supporting substrate are joined by thermocompression disclosed in Patent Document 2 or by a method of resin adhesion disclosed in Patent Document 3.
In describing in more in detail, the terahertz-wave detection element is generally formed in a relatively small size of about a few mm square to a few cm square in a planar view. Therefore, as described above, in order to improve manufacturing efficiency and secure accuracy of thinning the layer, the terahertz-wave detection element having the thin-layer electro-optic crystal is usually obtained by performing what is called a multi-piece forming as follows. The electro-optic crystal and the supporting substrate are respectively prepared as large-size mother substrates. Both mother substrates are joined together to obtain a joined body. The electro-optic crystal is thinned by mechanical polishing and the like. Finally, the joined body is cut into elements (chips) of desired sizes. Further, a film formation processing (a coating processing) in the case of providing a total reflection film and a reflection prevention film on the front and back surfaces of the detection element in order to improve detection efficiency is also usually performed to the mother substrates.
Moreover, in order to realize high spatial resolution, the terahertz-wave detection element needs to have excellent flatness and excellent parallelism. That is, it is necessary that the terahertz-wave detection element has small warping and small surface unevenness. When the flatness and parallelism of the terahertz-wave detection element used in the observation apparatus are poor, there occurs a phenomenon that an observation image is degraded or blurred, and satisfactory observation cannot be performed.
In the case of performing multiple piece forming as described above, a joined body of mother substrates before cutting needs to have excellent flatness and parallelism. For example, in order to obtain spatial resolution at least equal to or smaller than 20 μm, flatness equal to or smaller than 25 μm and parallelism equal to or smaller than 3 μm are necessary in the state after thinning the mother substrate of the electro-optic crystal, in terms of conversion to the joined body of a 4-inch diameter mother substrate. In order to realize such flatness and parallelism in the joined body, the mother substrate of the supporting substrate needs to satisfy these conditions of the flatness and parallelism.
However, conventionally, when flatness is high, there has been a problem in that an air bubble is included in the joined part at the time of joining the two mother substrates by resin adhesion.
When an air bubble exists in the joined part, in the process of thinning the mother substrate of the electro-optic crystal by polishing, the mother substrate is broken at a portion of the air bubble in some cases, and broken pieces scatter to good portions having no air bubble and form scratches. In this case, if the scratches are deep to such an extent that they cannot be removed by subsequent polishing, the total joined body becomes a defective product.
Further, when observation is performed by using a terahertz-wave detection element in which the air bubble exists on a joined surface between the electro-optic crystal and the supporting substrate, there arises a problem in that detection light is scattered or is irregularly reflected at the air bubble portion, and a change in the refractive index generated in the terahertz-wave electro-optic crystal cannot be detected in a high S/N ratio, and high spatial resolution cannot be obtained.